1. Technical Field
The disclosure relates generally to the synthesis of useful chemical byproducts and, more specifically, to the synthesis of ammonia using nano-size metal catalyst particles.
2. Related Art
Ammonia synthesis is an important industrial process. Ammonia is produced in huge quantities worldwide, for use in the fertilizer industry, as a precursor for nitric acid and nitrates for the explosives industry, and as a raw material for various industrial chemicals.
Despite an energy production cost of about 35 to 50 GJ per ton of ammonia, the Haber-Bosch process is the most widespread ammonia manufacturing process used today. The Haber-Bosch process was invented in the early 1900s in Germany and is fundamental to modern chemical engineering.
The Haber-Bosch process uses an iron catalyst to improve NH3 yields. Being a transition metal with partially occupied d-bands, iron represents a surface suitable for adsorption and dissociation of N2 molecules. An example of a commonly used iron catalyst is reduced magnetite ore (Fe3O4) enriched (“promoted”) with oxides of, for example, aluminum, potassium, calcium, magnesium, or silicon.
In the Haber-Bosch process, ammonia is synthesized using hydrogen (H2) and nitrogen (N2) gases according to the net reaction (N2+3H2→2NH3). The mechanism for iron-catalyzed ammonia synthesis is stated below in four dominant reaction steps, wherein “ads” denotes a species adsorbed on the iron catalyst and “g” denotes a gas phase species:N2(ads)→2N(ads)  (1)H2(ads)→2H(ads)  (2)N(ads)+3H(ads)→NH3(ads)  (3)NH3(ads)→NH3(g)  (4)The rate limiting step in the conversion of nitrogen and hydrogen into ammonia has been determined to be the adsorption and dissociation of the nitrogen on the catalyst surface. Thermodynamic equilibrium of the reaction is shifted towards ammonia product by high pressure and low temperature. However, in practice, both high pressures and temperatures are used due to a sluggish reaction rate. Due to overall low reaction efficiency when hydrogen and nitrogen are first passed over the catalyst bed, most ammonia production plants utilize multiple adiabatically heated catalyst beds with cooling between beds, typically with axial or radial flow. High pressure favors the adsorption process as well, but at a cost of increased operational and capital costs.
At pressures above 750 atm, there is an almost 100% conversion of reactants to the ammonia product. Because there are difficulties associated with containing larger amounts of materials at this high pressure, lower pressures of about 150 to 250 atm are used industrially. By using a pressure of around 200 atm and a temperature of about 500° C., the yield of ammonia is about 10 to 20%, while costs and safety concerns in the plant and during operation of the plant are minimized. Nevertheless, due in part to high pressures used in the process, ammonia production requires reactors with heavily-reinforced walls, piping and fittings, as well as a series of powerful compressors, all with high capital cost. In addition, generation of those high pressures during plant operation requires a large expenditure in energy.
In an effort to reduce the energy requirements of this process, the Kellogg Advanced Ammonia Process (KAAP) was developed using a ruthenium catalyst supported on carbon. The KAAP catalyst is reported to be 40% more active than the traditional iron catalysts. Use of this catalyst allowed the reactor pressure to be reduced, but the high cost of the precious metal ruthenium catalyst and the sensitivity of the catalyst to impurities in the hydrogen feed stock have prevented widespread use for ammonia synthesis. Other catalysts being studied include cobalt doped with ruthenium, but few encouraging results have been exhibited to date. Thus, after almost 90 years of ammonia synthesis, the Haber-Bosch process remains the most commonly used ammonia synthesis mechanism.
For the last 100 years, iron-based catalysts have been used in industrial ammonia synthesis. This catalyst is prepared by melting magnetite (Fe3O4) with a promoter compounds, for example potassium or calcium, and solidifying. The resulting porous material is then crushed into granules, generally in the size range of 1-10 millimeters. Active catalyst is then produced by reduction of iron oxides with hydrogen and nitrogen gas mixture, to give porous iron, and unreduced promoter oxides. Approximately 50% of this catalyst is void volume.
Improvements to Haber-Bosch catalysts focus on the addition of promoters for improved activity ammonia synthesis. U.S. Pat. Nos. 4,789,657 and 3,951,862 describe processes of preparing a magnetite-based ammonia synthesis catalyst via the melting of iron oxide with other compounds, such as Al2O3, K2O, CaO, MgO, and SiO2, and grinding into granules. U.S. Pat. No. 5,846,507 describes an iron composition having a non-stoichiometric oxide content and additional promoters, prepared by melting. Suggestions of reducing the processing pressures have been made, but have not been achieved economically.
Non-ferrous metal oxides may also be incorporated into the granules. For example, U.S. Pat. No. 6,716,791 describes the addition of cobalt and titanium oxides in a 0.1-3.0% weight ratio as additional promoters to aluminum, potassium, calcium, and magnesium. U.S. Pat. No. 3,653,831 describes the addition of platinum to improve reaction efficiency, however given the expense of platinum this may not be feasible at large scales. Other promoters, such as cerium described in U.S. Pat. No. 3,992,328 have also been shown to increase activity. Other improvements include alternative catalysts, such as those described in U.S. Pat. Nos. 4,163,775 and 4,179,407. These supported catalysts include ruthenium, rhodium, lanthanides and alloys.
Ideally, highly active ammonia catalysts can be used without significant changes to the many existing ammonia plants that exist today; the best candidates would be a “drop in” solution for existing manufacturers. Retrofit and reconstruction of these plants could be costly should there be a need to change design based on catalyst properties, such as space velocity. The best candidate catalyst would exhibit increased activity, have similar basic properties as compared to existing catalysts, and reduce operating costs. Non-ferrous catalysts in the above referenced prior art do not overcome all of these constraints because 1) catalyst cost increases more than catalyst efficiency, 2) the catalyst may not have the same properties that allow for seamless operation in existing ammonia production plants, or 3) the catalyst may have high activity but do not meet long term durability requirements.